Process for the manufacture of madrell{40 s salt

ABSTRACT

Production of Maddrell&#39;&#39;s salt by heating feed material consisting of monobasic sodium phosphate to a temperature substantially of 450* C., comprising carrying out the heating step in the temperature range between 200* and 450* C. while maintaining in the feed material a steam partial pressure between 50 and 450 mm. mercury.

United States Patent Kern et al.

1451 Mar. 28, 1972 PROCESS FOR THE MANUFACTURE OF MADRELL'S SALT [72] Inventors: Winfried Kern, Hunh near Cologne; Gero Heymer, Knapsack near Cologne; Heinz Harnisch, Lovenich near Cologne, all of Cie ma y Manama [73] Assignee: Knapsack Aktiengesellschaft, Knapsack near Cologne, Germany [22] Filed: June6,1968

[21] Appl. No.: 734,941

[30] Foreign Application Priority Data June 27, 1967 Germany ..K 62653 [51] Int. Cl. ..Clb /30 [58] Field of Search ..23/106, 106 A [56] References Cited UNITED STATES PATENTS 2,021,012 11/1935 McCullough 23/1 06A 2,356,799 8/ 1944 Taylor et al ..23/106 2,737,443 3/1956 Wright 3,063,801 11/1962 Groves ..23/l06 A OTHER PUBLICATIONS Van Wazer, Phosphorus and its Compounds, Vol. 1, pp. 665, 668 (Interscience Pub.- 1958) Perry, Chem. Engr s Handbook, 4th Ed., Sect. 20, pp. 31- 33 (McGraw Hill- 1963) Primary Examiner-Oscar R. Vertiz Assistant Examiner-Charles B. Rodman Attorney-Connolly and Hutz ABSTRACT Production of Maddrell's salt by heating feed material consisting of monobasic sodium phosphate to a temperature substantially of 450 C., comprising carrying out the heating step in the temperature range between 200 and 450 C. while maintaining in the feed material a steam partial pressure between and 450 mm. mercury.

981 31213 Dra Figures Production of Madclrclls salt by heating of M11 04501). under various steam parLiaL pressures ail/minus kealin nztes.

ZMuddmll PROCESS FOR THE MANUFACTURE OF MADRELL 'S SALT The present invention relates to a process for the manufacture of water-insoluble sodium polyphosphate, so-called Maddrells salt, by thermal treatment of monobasic sodium phosphate.

lt is known that Maddrells salt can be produced by dehydration of monobasic sodium phosphate. Depending on the reaction conditions used, the dehydration is found to en-. tail more or less the formation of water-soluble phosphates, such as acid sodium pyrophosphate, sodium trimetaphosphate or long chain soluble polyphosphates.

F. Thilo and R. Ratz in Zeitschrift fur anorganische und allgemeine Chemie," 258 (1949), page 53, have reported that Maddrells salt can be produced by heating Nal-l PQ, for some days to temperatures of 350 C. and freeing the resulting product later from soluble phosphates, which are present or have formed despite the long heating periods, by washing with water.

D.E.C. Corbridge and RR. Tromans (Analytical Chemistry 30 (1958), pages 1,101 1,110) also produce Maddrells salt by subjecting Nal-l PO to thermal treatment over days at 380 C. and washing later the resulting heat-treated product.

A commercial process for making Maddrells salt has been disclosed in US. Pat. No. 2,356,799, wherein NaH PQ, moistened with up to 5 water is compressed into pellets which are heated later in a stream of hot air to a temperature between 300 and 460 C., in a rotary kiln. The resulting products are formed of only 95 percent of Maddrells salt (cf. Journal of American Chemical Society, 81 (1959), page 79) and contain varying proportions of soluble phosphates, which are practically always above 1 percent by weight.

In Handbuch der praparativen anorganischen Chemie, published by Ferdinand Enke Verlag, Stuttgart, Brauer has reported that the production of Maddrells salt was sometimes found to fail completely for unknown reasons.

In many cases, it is desirable to produce Maddrells salt in a form fairly free from water-soluble phosphates, especially when the salt is intended for use as a polishing agent for tooth pastes. More particularly, it is desirable to obtainproducts containing less than 1 percent by weight of soluble phosphates, which are known to have strong complex-forming properties and, therefore, a deleterious effect upon tooth enamel.

It has now unexpectedly been found that the transformation of Nal-l PO into Maddrells salt is a function of the steam par tial pressure prevailing in the reaction material.

As shown in the following table, the transformation of NaH PO, into Maddrells salt is more especially a function of the steam partial pressure in the critical temperature range between 200 and 450 C.

TABLE I:

Heat Treatment of NaH PO at a Heating Rate of 15 C. Per Minute.

Steam partial pressure in mm. mercury, in the Steam partial pressure in mm. mercury, in the Weight percent Maddrells salt The balance to The present process for the production of Maddrells salt by heating feed material consisting of monobasic sodium phosphate to a temperature of 450 C. comprises more especially carrying out the heating step in the temperature range between 200 and 450 C. while maintaining in the feed material a steam partial pressure between 50 and 450 mm. mercury.

The steam partial pressure desired to prevail within the critical temperature range defined above can be produced by regulation of the heating rate and/or by the simultaneous passage of a gas poor in steam, particularly air, over the reaction gas.

The heating rate for the feed material should preferably be regulated by conveying the said feed material through a heating zone arranged to establish a suitable temperature curve therein. The heating rates should advantageously be selected lower than 50 C. per minute, more advantageously lower than 30 C. per minute.

it has also been found advantageous to use as the feed material NaH PQ, with a surface area of at least 0.1 square meter per gram, determined by the BET-method. This material can preferably be produced by the vacuum drying of NaH PQ, 2 H O or by the spray-drying of a Nal-l PO solution, more preferably by the drying of NaH PO 2 H O in vacuo at a pressure of less than mm. mercury. Still further, it has been found advantageous, in the temperature range between 200 and 450 C., to impart to the reaction material normally in horizontal motion an additional rotary motion, so as to accelerate steam diffusion.

Given that the steam partial pressure prevailing in the feed material is outside of the limits specified above, the NaH PO is found during the heat treatment to 450 C. to be transformed into Maddrells salt plus a considerable proportion of sodium trimetaphosphate.

In the following tabulated Examples, the Nal-IJO, was heated to 450 C. at heating rates of 15 C., 8 C. and 5 C. per minute under different steam partial pressures, and the proportion of Maddrells salt contained in the final products was determined by analysis.

The individual results obtained are graphically plotted in FIG. 1 of the accompanying graph.

TABLE ll:

Heating Steam partial Quantity ofMaddrells rate in pressure in suit in 71 by weight; C. per mm. mercury the balance to give minute I00 is formed of soluble phosphates.

salt to be obtained.

The Nal-I,PO used in all of the above examples had a surface of 0.25 square meters/gram, determined by the BET- method.

Maddrells salt should be conveniently produced in an apparatus such as that shown diagrammatically in the accompanying drawing, FIG. 2.

A kiln 2 meters long was provided with heating windings (not shown in the drawing) so as to establish therein a temperature curve such as that shown in the drawing, and small stainless steel boats filled with NaI'I PO were slowly conveyed through the kiln, at a constant velocity. Depending on the boat velocity, air was exhausted from the kiln, near I, so as to establish in the feed material a steam partial pressure within the range specified above.

EXAMPLE I The mean steam partial pressure was found to be l05 mm.

mercury.

The resulting final product (425 grams) was formed of 99.4 percent Maddrells salt and obtained in the form of a readily friable cake, easy to remove from the boats. The product contained 0.6 percent soluble matter.

EXAMPLE 2 500 grams NaI-I PO placed in small stainless steel boats, were conveyed through a kiln such as that shown in FIG. 2 at a velocity of 7 cm./minute, corresponding to a temperature in crease of 16 C./minute. Air was exhausted at a rate of 360 liters/hour and the steam partial pressure was 90 mm. mercury. The final product (425 grams) was formed of 99.1 percent Maddrells salt and readily friable. It contained 0.9 percent soluble matter.

Upon increasing the boat velocity, it was necessary to exhaust air at an overproportional rate. The reason for this was that the high heating rates prevented the steam from diffusing rapidly enough from the sample.

EXAMPLE 3 500 grams NaI-I PO were conveyed in the manner described in the preceding Examples through a kiln such as that shown in FIG. 2, at a constant velocity of 8 cm./minute,.

corresponding to a temperature increase of 185 C./minute. Air was exhausted at a rate of 440 liters/hour andthe steam partial pressure was 85 mm. mercury. The final product (425 grams) was formed of 96 percent Maddrells salt and 4 percent soluble matter.

Upon increasing the rate of exhausted air, it was found that even a heating rate of 185 C./minute still enabled final products with a minimum proportion of 99 percent Maddrells EXAMPLE 4 described in the preceding Examples through the kiln, at a velocity of 8 cin./minute., corresponding to a temperature increase of l8.5 C./minute. Air was exhausted at a rate of 800 liters/hour and the steam partial pressure was 50 mm. mercury.

The resulting final product was formed of 99.2 percent Maddrells salt and 0.8 percent soluble matter.

Upon exhaustion of too small a quantity of airor upon arresting the exhaustion, which resulted in a higher steam partial pressure outside of the preferred range, trimetaphosphate was fw to have formed even in the p aratus of FIG. 2

EXAMPLE 5 450 grams NaI-I PO placed in nine small stainless steel boats, were conveyed through the kiln at a velocity of 3.6 crnJminute, corresponding to a heating rate of 8.3 C./minute, the exhaustion of air being arrested. The first boat was found to contain substantially pure Maddrells salt (99 percent Maddrells salt plus 1 percent sodium trimetaphosphate). The second boat was found to contain strongly sintered final product, which contained no more than 77 percent Maddrells salt plus 23 percent sodium trimetaphosphate. The third and the following six boats contained very strongly sintered and fused final produets,'which were found to contain Maddrells salt in a proportion of 50 percent and even less.

The above experiment was repeated under inversed conditions, i.e., by heating NaI-l PO under too low a IMO-partial pressure, in the kiln of FIG. 2.

EXAMPLE 6 50 grams Nal-I,PO,, placed in a small stainless steel boat, I were conveyed through a kilnat a velocity of 1.8 cm./minute, corresponding to a temperature increase of 4.l C./minute. Air was exhausted at a rate of 2,600 liters/hour and the steam partial pressure was 2 mm. mercury. The final product was found to contain percent'Maddrells salt and 25 percent soluble matter.

It was also found as has already been mentioned above that NaI-I,PO samples with a large surface area, such as those obtained by vacuum or spray drying, were particularly useful for transformation into Maddrells salt.

EXAMPLE 7 NaI-I PQ, ZI-I O, which had been prepared by reaction of stoichiometric proportions of phosphoric acid percent by weight) and sodium hydroxide solution (50 percent by weight) followed by recrystallization, filtration and washing with alcohol, was freed from water of crystallization by treatment in a vacuum drying cabinet at C. at pressures of 10, 20, 40, 60, 80 and mm. mercury, respectively.

50 grams each of the resulting final products were placed in ,a small stainless steel boat and conveyed in the manner described in the preceding Examples through a kiln such as that shown in FIG. 2, at a velocity of 4.6 cm./minute, corresponding to a temperature increase of 10.6 C./minute, and

Surface 1 In percent by weight of Maddrells salt obtained from NaII PO4.

2 Not determinable.

We claim:

1. in the process for the production of Maddrells salt by heating monobasic sodium phosphate to temperatures up to about 450 C. in the presence of water vapor the improvementl which comprises passing the monobasic sodium phosphate; having a surface area of at least 0.1 square meter per gram through a heating zone, said heating zone having a tempera- 1 ture gradient increasing up to about 450 C., said monobasic? sodium phosphate being passed through said temperature gradient of said heating zone so as to be increased in tempera-1 ture of less than 50 C. per minute, reducing steam partiall pressure in said monobasic sodium phosphate of between 501 and 450 mm. mercury and recovering a water-insoluble, dry finely grained Maddrells salt.

2. The process of claim 1, wherein the said steam partial pressure within the critical tempe rature range between 200l and 450 C. is produced by regulating the heating rate.

3. The process of claim 2, wherein the heating rate is regulated by conveying the feed material through a heating zone having means for increasing the heating along the length of the I Z0116. n V V V V V 4. The process of claim 1, wherein the steam partial pres- ;sure desired to prevail within the critical temperature range is produced by conveying a gas poor in steam over the feed material.

5. The process of claim 4, wherein the gas poor in steam is air.

6. The process of claim 1, wherein the feed material is j heated at a heating rate of less than 30 C. per minute.

7. The process of claim 1, wherein the feed material is vacuum-dried Nal'l PO 2 H O 8. The process of claim 7, wherein the feed material is Nal-l PQ, 2 H O vacuum-dried at a pressure of less than mm. mercury.

9. The process of claim 1, wherein the feed material is a spray-dried NaH PO -solution.

10. The process of claim 1, wherein, within the temperature range between 200 and 450 C., the reaction material normally in horizontal motion is imparted an additional rotary motion so as to accelerate steam diffusion. 

2. The process of claim 1, wherein the said steam partial pressure within the critical temperature range between 200* and 450* C. is produced by regulating the heating rate.
 3. The process of claim 2, wherein the heating rate is regulated by conveying the feed material through a heating zone having means for increasing the heating along the length of the zone.
 4. The process of claim 1, wherein the steam partial pressure desired to prevail within the critical temperature range is produced by conveying a gas poor in steam over the feed material.
 5. The process of claim 4, wherein the gas poor in steam is air.
 6. The process of claim 1, wherein the feed material is heated at a heating rate of less than 30* C. per minute.
 7. The process of claim 1, wherein the feed material is vacuum-dried NaH2PO4 . 2 H2O.
 8. The process of claim 7, wherein the feed material is NaH2PO4 . 2 H2O vacuum-dried at a pressure of less than 80 mm. mercury.
 9. The process of claim 1, wherein the feed material is a spray-dried NaH2PO4-solution.
 10. The process of claim 1, wherein, within the temperature range between 200* and 450* C., the reaction material normally in horizontal motion is imparted an additional rotary motion so as to accelerate steam diffusion. 